This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art. Communication systems are under constant demand to increase their ability to convey greater amounts of information both reliably and at ever increasing transfer rates. In order to enhance optical fiber capacity for metro and core network transmission, coherent optical systems with high-order quadrature-amplitude-modulation (QAM) signaling have attracted greater interest. While denser constellations lead to higher spectral efficiency, the signal becomes more sensitive to amplified spontaneous emission noise and laser phase noise, setting new challenges for receiver digital signal processing subsystems. Additionally, some computational schemes may use feedback processing in the coherent optical receiver.